Primitives of volume forms in Carnot groups

Annalisa Baldi, Bruno Franchi, Pierre Pansu

In the Euclidean space it is known that a function $f\in L^2$ of a ball, with vanishing average,is the divergence of a vector field $F\in L^2$ with$$\| F\|\_{ L^2(B)} \le C \|f\|\_…